Wow, I'm shocked at the negative attitudes in this thread. Porn and gambling are legit businesses, even if you don't like it. (And some people used to argue that part of the rise of the PC was because some people bought it as a "porn machine.")
It's important to keep these things (almost) in the open, because when they become illegal, criminals move in and people get hurt.
When I was an intern at a big-name, conservative company, one of my friends came from a porn website.